This invention relates to circuit isolation devices and especially to devices for isolating a voltage signal that is "floating" at some potential, from another electrical device, such as, an electrical instrument. Normally, electrical instruments must be operated at ground potential for safety reasons.
More particularly the invention relates to circuit isolators that use:
(1) a transformer to convert one initial voltage signal floating at some potential (upon modulation) to a representative voltage signal (upon demodulation) at ground potential; and
(2) photo responsive switch means such as photo field effect transistors (FETs) for modulating the initial voltage signal so that the control circuit for operating the modulating and demodulating circuits may be at ground potential.
It is frequently desirable to observe and/or measure voltage wave forms that are offset or "floating" by a significant voltage from ground. This is commonly accomplished by disconnecting the safety ground of the oscilloscope or other instrument being used, and allowing the instrument to float above ground potential. This practice is hazardous as accidental contact with the instrument case can result in lethal shock.
The types of electrical devices that need isolation of the type to which the present invention is directed include current shunts, physiological and medical instruments, and oscillographs and oscilloscope-type instruments.
Shunts (low value resistors), are often used for the measurement of DC current in applications such as power supplies, rectifiers, battery chargers, DC motor drives, inverters and battery plants. In many applications, the shunt voltage is to be monitored by a digital meter, or a current sensing and/or control device. In such applications there is frequently a need for the shunt to be "floating" at a different potential from ground than the instrumentation or control circuitry.
In the case of physiological and medical instruments, electrical isolation is essential for patient protection. Also, physiological signals are of very low voltage, and it is essential that the instrumentation used not introduce noise.
In the case of oscillographs and oscilloscopes, there are a wide range of applications where isolation may be desirable. These applications include power system monitoring, machine vibration monitoring, mechanical shock and vibration testing, and others.
As the need for such electrical devices has increased due to the expanding use of sophisticated power supply inverters, motor drives and the like, several devices have been introduced that isolate the voltage signal from the instrument thus allowing for safer utilization. For example, a transformer can provide isolation and a high degree of accuracy at medium to high frequencies. At low frequencies however the size of the transformer (i.e. the core area) must be increased in order to avoid saturation and this reduces accuracy at high frequencies. Transformer isolators cannot be used at all when the input signal has a DC component.
Photo couplers can also be used but these devices have wide variations in gain and poor linearity. Also, these devices are relatively slow so that their application is limited to the lower frequencies.
Isolators presently in use are large and expensive. The Tektronix AG902A, for example is a two channel isolator having a volume of about 2/3 cu. ft. and a weight of over 13 lbs. This device is larger, heavier and more expensive than many of the oscilloscopes it would be used with. Because of this inconvenience, the device is often too cumbersome to use.
The Tektronix device uses both transformer and optical coupling techniques to achieve a DC to 20 mhz band width. Photocouplers are used at low frequencies, transformers are used at high frequencies, and both circuits are used at midfrequencies where the outputs of both the transformers and the photocouplers are added together. This combination of outputs is difficult to adjust for uniform gain over a wide frequency range.
Isolator circuits are also found in isolation amplifiers. Typical isolation circuits for these amplifiers are found in the following U.S. patents:
U.S. Pat. No. 3,988,690
U.S. Pat. No. 4,066,974
U.S. Pat. No. 4,163,950
U.S. Pat. No. 4,191,929
The device of the present invention satisfies the problems and shortcomings identified above and affords other features and advantages heretofore not obtainable.